Readout system for a full-color image input scanner having three linear arrays of photosensors

ABSTRACT

In a full-color image scanner having three parallel linear arrays of photosensors, each linear array of photosensors is sensitive to one primary color. With each readout cycle relating to a row of small areas on an original image moving past the scanner, the middle array of the three arrays of photosensors reads out its video data before the other two. This technique allows the arrays of photosensors to be spaced relatively close to each other, and in turn the close spacing of photosensors requires relatively fewer scan lines of video data to be temporarily buffered in the readout process.

FIELD OF THE INVENTION

The present invention relates to image sensor arrays having multiplerows of photosensors, each row of photosensors being sensitive to aparticular primary color. Such image sensor arrays are used, forexample, for full-color scanning of hard-copy original images.

BACKGROUND OF THE INVENTION

Image sensor arrays typically comprise a linear array of photodiodeswhich raster scan an image-bearing document and convert to themicroscopic image area viewed by each photodiode to image signalcharges. Following an integration period, the image signals areamplified and transferred to a common output line or bus throughsuccessively actuating multiplexing transistors.

U.S. Pat. No. 5,148,268 discloses a typical arrangement of a full-colorimage sensor array. Separate linear arrays of photosensors are arrangedin parallel on a single bar, with the photosensors in each linear arraybeing provided with a filter thereon of one primary color. The bar iscaused to move relative to an original image in a scan direction whichis generally perpendicular to the direction of the arrays. As the sensorbar moves along the original image, each portion of the area of theoriginal image is exposed to each of the linear arrays of photosensorsin sequence. As each array of photosensors moves past a particular smallarea in the original image, signals according to the different primarycolor separations of that area are output by one of the photosensors ineach array. In this way three separate sets of signals, each relating toone primary color, are produced by the linear arrays of photosensors.

An important parameter in the design of an image sensor array is theresolution of the array, which will of course affect the quality ofimage signal based on an original image. One type of resolution isdictated by the physical configuration of the individual photosensorsalong the array: the higher the number of individual photosensors withina given unit of length along the array, the higher the possibleresolution of data that may be output by the array. This "fast scan" orx-direction resolution is of course fixed by the size and spacing of thephotosensors in the array.

Another type of resolution associated with an array is the "slow-scan,"or y-direction, resolution, which is the resolution of the image alongthe direction perpendicular to the direction of the array, which wouldbe the direction of an original image moving relative to the array. Incontrast to the x-dimension resolution, which is fixed by the physicalcharacteristics of the array, the y-direction resolution is determinedby the speed of an original image relative to the array, coupled withthe integration times of individual photosensors. If the original imageis moving relative to the array at a constant velocity, and thephotosensor is operating at a high speed, each integration time of thephotosensor will cause exposure to a relatively small area on theoriginal image; if the integration time is longer, with each integrationtime an individual photosensor will be "looking at" a relatively largerarea of the original image. In brief, the shorter the integration timeof an individual photosensor in the array, the higher the y-directionresolution of the array.

As will be described in detail below, a technical complication mayresult where the desired y-direction resolution, which is related to theintegration times in an array, is different from the inherentx-direction resolution of the array. For example, one possible designfor a full-page-width full-color array provides, by virtue of itsphotosensor size, a fixed 400 SPI resolution in the x-dimension, but canprovide, by virtue of the operational speed of the photosensors, a 600SPI resolution in the y-direction. The present invention is directed tophysical and operating parameters of a full-color scanning array whichovercomes certain design requirements caused by high y-directionresolution.

DESCRIPTION OF THE PRIOR ART

U.S. Pat. No. 5,148,268, referenced above, and U.S. Pat. No. 5,543,838disclose multiplexing systems for reading out signals from a full-colorimage sensor bar having three linear arrays of photosensors, each lineararray having a filter thereon for one primary color.

U.S. Pat. No. 5,519,514 discloses a full-color sensor bar having threeparallel arrays of photosensors, each array being sensitive to adifferent primary color. With each scan cycle as the bar is movedrelative an original image, the exposure of the photosensors isprecisely timed so that the optical "center of gravity" for each exposedarea in the original image is superimposed for all of the primary colorphotosensors.

U.S. Pat. No. 5,550,653 discloses a full-color input scanner, which isoperated in a mode adapted for efficient scanning of documents withsimple color relations. Three linear arrays of photosensors, each lineararray corresponding to one primary color, are moved relative to theoriginal image. One linear array of photosensors operates on a fullcycle and converts every single scan line of the original image intodigital signals. Simultaneously, the other primary-colorsensitive lineararrays operate on half cycles and record only signals corresponding toan evenly-distributed subset of small areas of the original image.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, there is provided amethod of operating an image sensor for deriving image data from anoriginal image on a sheet. There is disposed on the image sensor a firstlinear array of photosensors, a second linear array of photosensors, anda third linear array of photosensors, the second linear array ofphotosensors and the third linear array of photosensors being parallelto the first linear array of photosensors. The second linear array ofphotosensors is disposed between the first linear array of photosensorsand the third linear array of photosensors. The sheet is moved at apredetermined velocity relative to the image sensor, in a processdirection perpendicular to the first linear array of photosensors. Asthe sheet moves relative to the image sensor, image data from the firstlinear array of photosensors, the second linear array of photosensors,and the third linear array of photosensors is periodically recorded,whereby for a small area on the sheet, the image data related to thesmall area is recorded by the second linear array of photosensors beforethe image data related to the small area is recorded by the first lineararray of photosensors or the third linear array of photosensors.

According to another aspect of the present invention, there is providedan image sensor for deriving image data from an original image on asheet. A first linear array of photosensors and a second linear array ofphotosensors is disposed parallel to the first linear array ofphotosensors, each photosensor in the first linear array of photosensorsand the second linear array of photosensors having a predeterminedlength along a process direction perpendicular to the first linear arrayof photosensors. A center of a photosensor in the first linear array isspaced from a center of a photosensor in the second linear array alongthe process direction by 14/9 a length of a photosensor in the firstlinear array along the process direction.

According to another aspect of the present invention, there is providedan image sensor for deriving image data from an original image on asheet. A first linear array of photosensors and a second linear array ofphotosensors is disposed parallel to the first linear array ofphotosensors, each photosensor in the first linear array of photosensorsand the second linear array of photosensors having a predeterminedlength along a process direction perpendicular to the first linear arrayof photosensors. A center of a photosensor in the first linear array isspaced from a center of a photosensor in the second linear array alongthe process direction by 10/9 a length of a photosensor in the firstlinear array along the process direction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the relationship between a full-color imagesensor chip recording an image on a sheet passing relative thereto, andthe resulting video data being sent to a buffer;

FIG. 2 shows the relationship of recording small areas on a sheet withthree primary-color-sensitive photosensors, according to a method knownin the prior art;

FIG. 3 is a diagram similar to that of FIG. 2, showing the recording ofsmall areas on an original image in a manner having a relatively highresolution in the slow-scan direction according to one embodiment of thepresent invention; and

FIG. 4 is a diagram similar to that of FIG. 2, showing the recording ofsmall areas on an original image in a manner having a relatively highresolution in the slow-scan direction according to another embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

For purposes of the description and claims, the following terms shall behere defined:

Scan line: the complete color data from a single linear array ofphotosensors (such as for one primary color sensitive linear array ofphotosensors) for a given integration or exposure period, or the datacollected in the fast-scan or x-direction.

Scan line pitch: the distance a photosensor moves with respect to theoriginal image between the recording of scan lines in the slow-scan ory-direction.

Pixel pitch: the distance between the center of one color pixel(effective photosensitive area of a photosensor) to the center of thenext color pixel opening in the y-direction.

Pixel size: the length of a photosensor along the y-direction.

FIG. 1 is a diagram showing the relationship of an example portion of afull-color photosensor array recording digital data based on an originalhard copy image on a sheet. A chip indicated as 10 represents all orpart of a full-color sensor bar; as in known in the art, such a sensorbar may include a single chip, which is exposible to a page-width imagethrough reduction optics, or could be one chip in a butted array ofsimilar chips in a full-page-width array. The chip 10 includes thereonthree rows of photosensors, here indicated as 12R (Red), 12G (Green),and 12B (Blue). As is familiar in the art, each row of photosensors ismade sensitive to one primary color, such as by including a translucentfilter thereon (not shown).

An original image which is desired to be converted into digital data isborne on a sheet, an outline of a portion of which is shown in phantomin FIG. 1 as S. As is familiar in the art of hard-copy scanning, sheet Sis moved at a predetermined constant velocity in a process direction,here indicated as D, which is perpendicular to the direction ofextension of each row of photosensors 12R, 12G, and 12B. In this way, aparticular small area on sheet S, such as the example small areaindicated as A in FIG. 1, is sequentially exposed to one photosensor foreach primary color in the array. As the small area A of the originalimage moves past each primary-color-sensitive photosensor, its colorproperties are recorded for each primary color in sequence. A row ofsmall areas A must pass over each of the three linear arrays ofphotosensors 12R, 12G, and 12B, for the color properties of the row ofsmall areas to be recorded by the photosensors.

In a practical embodiment of a full-color array, there is typically asingle output line or equivalent thereof for each individual row, 12R,12G, 12B, of photosensors, and at a particular time, there will beoutput from chip 10 relatively long series of video signals,corresponding to, for example, all of the red signals in a scan line,then all the green signals in the scan line, and then all the bluesignals. However, for the video signals to be sent on toimage-processing circuitry, the image data should be arranged on apixel-by-pixel basis, so that the primary color signals RGB for everyindividual small area A are together: thus, the sets of separate redsignals, green signals, and blue signals for every scan line must betemporarily stored and then rearranged on a pixel-by-pixel basis. Thus,there is shown in FIG. 1 a buffer 14, which accepts data relating tocomplete scan lines for rows 12R, 12G, and 12B, temporarily stores theimage data corresponding to these scan lines, and then reads out thedata as individual RGB signals, one set of signals for each pixel-sizesmall area such as A.

The buffer 14 must hold the video data for a particular scan line untilall of the primary color data for a scan line is collected. Forinstance, if data is being read out in the order red scan line, greenscan line, blue scan line, the buffer 14 must hold the red video dataand green video data for a scan line until it finally receives the bluevideo data for the scan line (i.e., only after the row of small areas Aof the original image in question is moved past the blue photosensors12B, and the resulting blue video signals read out of chip 10): onlythen would the full three-color signals for the scan line be completefor the pixels in the row and the data be able to be read out of thebuffer 14 and sent on to image-processing circuitry. Generally it isdesirable to minimize the amount of temporary storage of video signalsin buffer 14, as the buffer is a memory which is a manufacturing aexpense, and the buffering step can delay the response of the chip 10and the entire scanning system.

FIG. 2 is a diagram which shows the space-time relationship ofprimarycolor-sensitive photosensors in a basic case known in the priorart, where the x-direction and y-direction resolutions are intended tobe equal. On the left of the diagram in FIG. 2 are shown a red, green,and blue photosensor. A portion of a sheet S, such as shown by thedotted lines on the right of the Figure, show the location of anindividual small areas such as A in FIG. 1 relative to the photosensorsas the sheet moves in a direction D over time, in sequence from left toright. In this particular embodiment, a portion of every cycle ofreading out a scan line of video signals from the various rows ofphotosensors is required for transfer of the resulting signals on avideo line out of the chip (for a fuller explanation of readout systems,particularly in a CMOS-type image scanner, reference can be made to thepatents referenced above). In order to take into account this necessaryportion of the cycle for readout of signals from each row ofphotosensors, there can be provided between centers of adjacentphotosensors in the y-direction an additional 1/3 pitch: as can be seenin FIG. 2, the distance between the centers of, for example, the R and Gphotosensors is 4/3 the length of each photosensor in the y-direction.In this 4/3, 3/3 of the pitch is the length of a photosensor itself, andthe additional 1/3 represents spacing which, when the sheet S moves inconstant velocity D relative to the photosensors, allows the necessaryone-third duty cycle for readout of the video data for each scan line.

With reference to the right hand portion of FIG. 2, it will be notedthat as the original hard copy image moves through direction D relativeto the photosensors, a number of scan lines must be recorded by thephotosensors before all of the color separations for a particular smallarea are recorded. Thus, as mentioned above, at least two scan lines ofdata must be stored temporarily until the final color is ready in thethird storage line. Generally, because of the extra 4/3 pitch of spacebetween the rows of photosensors, in this embodiment there must in factbe stored three lines of scan line data in buffer 14.

In this basic example of FIG. 2, and with reference to the terms definedabove, the spacing of primary-color photosensors is 4/3 of a scan linepitch; that is, 4/3 the distance the sheet S moves in direction Dbetween readout of scanline data from a row of photosensors for aparticular color. Significantly, the scan line pitch is determined bythe operating speed of the scanner: if the scan line data were read outat a faster rate with the motion of the sheet held constant, the scanline pitch would be shorter, because the sheet will have traveled ashorter distance between recording steps. In the FIG. 2 example, becausethe x-direction resolution is intended to be equal to the y-directionresolution, the scan line pitch happens to be equal to the pixel size,as those terms are defined above. A practical complication results,however, if the y-direction resolution is desired to be higher than thex-direction resolution, such as when an array having photosensors sizedfor 400 spi resolution is run at a high scan rate to achieve 600 spiresolution in the y-direction.

FIG. 3 is a diagram analogous to that of FIG. 2, showing the situationof recording various small areas on an original image in a situationwhere the y-direction resolution of the scanner is intended to begreater than the x-direction resolution. Because of the higher readoutrate in the y-direction, the integration time of each individualphotosensor is made smaller in the y-direction, and thus each individualphotosensor "looks at" an area on the original image which is smaller inthe y-direction: compare, for example, the dimensions of the small areasin FIG. 3, which are shorter in the y-direction, to the relativelysquare small areas in FIG. 2. With reference to the definitions givenabove, whereas the pixel size in a 400 spi image scanner is 63.5micrometers in the y-direction, because in the FIG. 3 example theoriginal image moves a shorter distance per scan line interval, the scanline pitch for the FIG. 3 example is only 42.5 micrometers. (Also,because it is desirable to have generally square photosensors, the widthof each photosensor in the x-direction should be close to 63.5micrometers, for a 400 spi resolution in the x-direction.)

In the FIG. 3 example, to provide the necessary center-to-center-spacingof 4/3 times 42.5 micrometers (the scan line pitch) would not bepossible because, in order to facilitate the 4/3 spacing with 400 SPIsensors operating at 600 SPI, the 400 SPI sensors would have to overlapin the y-direction. In the FIG. 3 embodiment, however, this necessaryspacing is accommodated by adding the equivalent of a whole extra scanline pitch to the 4/3 spacing, for a total spacing between centers ofadjacent primary-color photosensors of 1+4/3 scan line pitches, or 7/3scan line pitches. Thus, according to one aspect of the presentinvention, for an image scanning array having a resolution of 400 SPI inthe fast scan direction and an operating resolution of 600 SPI in theslow-scan direction (or a ratio of 1.5), the center-to-center spacingbetween adjacent photosensors in the array is 7/3 times the distance thesensor moves with respect to the original image between scan linereadouts in the slow scan direction. With regard to border-to-borderspacing of adjacent photosensors in this embodiment, as shown, thedistance between adjacent edges of such photosensors is 5/9 the lengthof each photosensor in the y-direction (making a center-to-centerspacing of 14/9 the length of each photosensor in the y-direction), or5/6 the scan line pitch.

One disadvantage of having relatively large spacing between adjacentrows of photosensors is that temporary storage of more scan line datawill be required in data buffer 14. In the example of FIG. 3, it wouldbe necessary to retain video data for five scan lines at a time, untilall of the primary color data for a particular small area isaccumulated. Once again, this takes into account not only the timerequired for a particular small area to pass by a photosensor for eachprimary color (because the photosensors are farther apart, one will haveto wait longer for an area to be "looked at" by every photosensor), butalso for the video data for each primary color to be read out into thedata buffer 14. It will be apparent that further whole pixel pitchescould be added to the spacing, such as for a center-to-center spacing of10/3 or 13/3, etc., but then even more storage of data would be neededin buffer 14. It would be more desirable to have a system which did notrequire an unusually large spacing between adjacent rows ofphotosensors, so that, among other reasons, not so many lines of videodata would have to be retained in the data buffer 14.

FIG. 4 shows a configuration of photosensors, combined with a readoutroutine, according to another aspect of the present invention. Onceagain, as in the FIG. 3 example, photosensors having the pixel sizesuitable for 400 SPI resolution are operated at a speed consistent witha 600 SPI resolution in the y-direction. However, according to thisaspect of the present invention, the center to-center spacing ofadjacent photosensors is only 5/3 the scan line pitch, as opposed to the7/3 times the scan line pitch of the FIG. 3 example. This closer spacingof adjacent rows of photosensors not only reduces the amount of "realestate" required on a chip such as 10 for the photosensors, but alsonecessitates fewer scan lines of temporary data storage in data buffer14: in a practical embodiment, the scheme of FIG. 4 requires temporarystorage of only four lines of video data, as opposed to the five linesin the FIG. 3 embodiment. With regard to border-to-border spacing ofadjacent photosensors in this embodiment, as shown, the distance betweenadjacent edges of such photosensors is 1/9 the length of eachphotosensor in the y-direction (thus making a center-to-center spacingof 10/9 the length of each photosensor in the y-direction), or 1/6 thescan line pitch.

This closer spacing of adjacent rows of photosensors in the FIG. 4embodiment is facilitated by a unique readout routine: according to thepresent invention, for every row of small areas on a sheet S movingrelative to the photosensors, in sequence the green scan line of videodata is read out first, then followed by the blue and red video data forthe same small areas in the row. More generally, if there is provided ona scanning array three parallel linear arrays of photosensors, and videodata from one particular linear array can be recorded at a predeterminedtime, according to the present invention for every small area on anoriginal image being scanned, the data from the middle linear array isrecorded prior to recording image data from either of the other lineararrays of photosensors. The configuration of recording small areas asshown in the sequence of FIG. 4 is what results from this technique.

According to this scheme, the rows of photosensors are placed 5/3 scanline pitches apart, and the readout order of video scan lines is G,B,Ror G,R,B. If the G, B, R order is used, the original image must bescanned in the opposite orientation to the direction shown in theFigure.

As shown in the Figure, the green photosensor is read out first and thevideo data from row 2 of small areas on the original image is read out.By the time the blue video data is read out, the document has moved 1/3of a 600 SPI scan line and reads out row 4 of small areas on theoriginal (row 2+1 2/3 pitch +4/3 scan line movement). By the time thered video data is read out, the original image has moved 2/3 scan lineand is now reading out row 1 of small areas in the image document (row2-1 2/3 pitch +2/3 scan line movement). Since the data from one scanline contains colors from four different physical scan lines of theoriginal image, it can be seen from the Figure that only four lines ofvideo data are required to assemble colinear RGB pixel data for a row ofsmall areas on the original image.

With specific regard to the embodiment of FIG. 4 it should be noted thatthe resulting pattern of recording of different small areas (i.e.,within a row of small areas within a column of small areas which move insequence over a single set of R, G, B photosensors 12) relate to theread out of a scan line of image data from a large set of photosensors.Once again, in a typical commercial design of a CMOS-based image sensorchip, there exists in each linear array of photosensors 248 photosensorswhich output a scan line of video image data with every readout. Thus,with reference to the "time line" at the bottom of FIG. 4, between thereading out of 0 and 1/3 scan lines individual image signals from all248 green photosensors in the green linear array are read out, and froma 1/3 to 2/3 scan line time period, all of the blue photosensors areread out, and finally from 2/3 to 1 full scan line of time line, all ofthe red photosensors are read out. In the most common embodiment of aCMOS-based image sensor chip, such as described in U.S. Pat. No.5,148,268 referenced above, the read out period for an individualsingle-color linear array of photosensors immediately follows theintegration time (i.e., the recording step) of the linear array ofphotosensors, although it is possible that, in alternate designs ofimage sensors, the image sensor data may be held for some period of timebetween integration time and readout.

Nonetheless, consistent with FIG. 4, with the passage of time, it can beseen, for each triplet of R, G, B photosensors, the middle greenphotosensor records first in time, immediately reads out its image data(along with the image data of the other 247 active green photosensors inthe linear array), then the blue photosensors record and read out, andthen finally the red photosensors record and read out. This sequence hasthe effect of recording for the middle linear array of photosensors withregard to each small area on the original image, as is claimed below. Inthis embodiment, the buffering of scan line data on a continuous basisin four storage lines of data is as follows:

Data Stream out of Sensor Chip (Color_(column) x, row y)

Storage Line 1: G₁,2, G₂,2, G₃,2, . . . , B₁,4, B₂,4, B₃,4, . . . ,R₁,1, R₂,1, R₃,1, . . .

Storage Line 2: G₁,3, G₂,3, G₃,3, . . . , B₁,5, B₂,5, B₃,5, . . . ,R₁,2, R₂,2, R₃,2, . . .

Storage Line 3: G₁,4, G₂,4, G₃,4, . . . , B₁,6, B₂,6, B₃,6, . . . ,R₁,3, R₂,3, R₃,3, . . .

Storage Line 4: G₁,5, G₂,5, G₃,5, . . . , B₁,7, B₂,7, B₃,7, . . . ,R₁,4, R₂,4, R₃,4, . . .

As can be seen in the above chart, four lines of image data are requiredbefore the G, B, and R image data for a particular column x, row y onthe original image is accumulated: See for example, datum B₁,4 at the"front" (or from the first line) of the buffer and datum R₁,4 at the"back" (or from the last line) of the buffer.

In summary, the advantage of the configuration of the FIG. 4 embodiment,wherein the middle linear array is read out first for every row of smallareas in the original image, is that the parallel rows of photosensors,each for one primary color, can be spaced relatively close to eachother, 5/3 scan line pitches, versus 7/3 scan line pitches in the FIG. 3embodiment. This closer spacing not only saves a fairly significantamount of space on a chip including the scanning array, but, by virtueof the closer photosensor rows being more compressed in time in ascanning process, fewer scan lines of video data need be temporarilyretained to assemble all of the primary color data for a row of pixelsin the original image. Once again, in a practical embodiment, theconfiguration of FIG. 4 requires the temporary buffering of four scanlines of video data, while the less-closely-spaced photosensor rows ofthe FIG. 3 embodiment require at least five scan lines of video data tobe temporarily buffered.

While the invention has been described with reference to the structuredisclosed, it is not confined to the details set forth, but is intendedto cover such modifications or changes as may come within the scope ofthe following claims.

What is claimed is:
 1. A method of operating an image sensor forderiving image data from an original image on a sheet, comprising thesteps of:providing on the image sensor a first linear array ofphotosensors, a second linear array of photosensors, and a third lineararray of photosensors, the second linear array of photosensors and thethird linear array of photosensors being parallel to the first lineararray of photosensors, the second linear array of photosensors beingdisposed between the first linear array of photosensors and the thirdlinear array of photosensors, a center of a photosensor in the firstlinear array being spaced from a center of a photosensor in the secondlinear array along the process direction by 5/3 times a scan line pitch;causing the sheet to move at a predetermined velocity relative to theimage sensor, in a process direction perpendicular to the first lineararray of photosensors; as the sheet moves relative to the image sensor,periodically recording image data from the first linear array ofphotosensors, the second linear array of photosensors, and the thirdlinear array of photosensors, whereby for a small area on the sheet, theimage data related to the small area is recorded by the second lineararray of photosensors before the image data related to the small area isrecorded by the first linear array of photosensors or the third lineararray of photosensors.
 2. The method of claim 1, the length of aphotosensor in the first linear array along the process direction beinggreater than one scan line pitch.
 3. The method of claim 1, the lengthof a photosensor in the first linear array along a directionperpendicular to the process direction being greater than one scan linepitch.
 4. The method of claim 1, the first linear array of photosensorsbeing sensitive to a first color and the second linear array ofphotosensors being sensitive to a second color different from the firstcolor.
 5. A method of operating an image sensor for deriving image datafrom an original image on a sheet, comprising the steps of:providing onthe image sensor a first linear array of photosensors, a second lineararray of photosensors, and a third linear array of photosensors, thesecond linear array of photosensors and the third linear array ofphotosensors being parallel to the first linear array of photosensors,the second linear array of photosensors being disposed between the firstlinear array of photosensors and the third linear array of photosensors,a distance between one edge of a photosensor in the first linear arrayand an adjacent edge of a photosensor in the second linear array being1/9 a length of a photosensor in the first linear array along theprocess direction; causing the sheet to move at a predetermined velocityrelative to the image sensor, in a process direction perpendicular tothe first linear array of photosensors; as the sheet moves relative tothe image sensor, periodically recording image data from the firstlinear array of photosensors, the second linear array of photosensors,and the third linear array of photosensors, whereby for a small area onthe sheet, the image data related to the small area is recorded by thesecond linear array of photosensors before the image data related to thesmall area is recorded by the first linear array of photosensors or thethird linear array of photosensors.
 6. The method of claim 5, the lengthof a photosensor in the first linear array along the process directionbeing greater than one scan line pitch.
 7. The method of claim 5, thelength of a photosensor in the first linear array along a directionperpendicular to the process direction being greater than one scan linepitch.
 8. The method of claim 5, the first linear array of photosensorsbeing sensitive to a first color and the second linear array ofphotosensors being sensitive to a second color different from the firstcolor.
 9. A method of operating an image sensor for deriving image datafrom an original image on a sheet, comprising the steps of:providing onthe image sensor a first linear array of photosensors, a second lineararray of photosensors, and a third linear array of photosensors, thesecond linear array of photosensors and the third linear array ofphotosensors being parallel to the first linear array of photosensors,the second linear array of photosensors being disposed between the firstlinear array of photosensors and the third linear array of photosensors,a distance between one edge of a photosensor in the first linear arrayand an adjacent edge of a photosensor in the second linear array being1/6 of a scan line pitch; causing the sheet to move at a predeterminedvelocity relative to the image sensor, in a process directionperpendicular to the first linear array of photosensors; as the sheetmoves relative to the image sensor, periodically recording image datafrom the first linear array of photosensors, the second linear array ofphotosensors, and the third linear array of photosensors, whereby for asmall area on the sheet, the image data related to the small area isrecorded by the second linear array of photosensors before the imagedata related to the small area is recorded by the first linear array ofphotosensors or the third linear array of photosensors.
 10. The methodof claim 9, the length of a photosensor in the first linear array alongthe process direction being greater than one scan line pitch.
 11. Themethod of claim 9, the length of a photosensor in the first linear arrayalong a direction perpendicular to the process direction being greaterthan one scan line pitch.
 12. The method of claim 9, the first lineararray of photosensors being sensitive to a first color and the secondlinear array of photosensors being sensitive to a second color differentfrom the first color.
 13. An image sensor for deriving image data froman original image on a sheet, comprising:a first linear array ofphotosensors and a second linear array of photosensors parallel to thefirst linear array of photosensors, each photosensor in the first lineararray of photosensors and the second linear array of photosensors havinga predetermined length along a process direction perpendicular to thefirst linear array of photosensors; a center of a photosensor in thefirst linear array being spaced from a center of a photosensor in thesecond linear array along the process direction by 14/9 a length of aphotosensor in the first linear array along the process direction. 14.The image sensor of claim 13, the first linear array of photosensorsbeing sensitive to a first color and the second linear array ofphotosensors being sensitive to a second color different from the firstcolor.
 15. The image sensor of claim 13, a distance between one edge ofa photosensor in the first linear array and an adjacent edge of aphotosensor in the second linear array being 5/9 a length of aphotosensor in the first linear array along the process direction. 16.An image sensor for deriving image data from an original image on asheet, comprising:a first linear array of photosensors and a secondlinear array of photosensors parallel to the first linear array ofphotosensors, each photosensor in the first linear array of photosensorsand the second linear array of photosensors having a predeterminedlength along a process direction perpendicular to the first linear arrayof photosensors; a center of a photosensor in the first linear arraybeing spaced from a center of a photosensor in the second linear arrayalong the process direction by 10/9 a length of a photosensor in thefirst linear array along the process direction.
 17. The image sensor ofclaim 16, a distance between one edge of a photosensor in the firstlinear array and an adjacent edge of a photosensor in the second lineararray being 1/9 a length of a photosensor in the first linear arrayalong the process direction.
 18. The image sensor of claim 16, the firstlinear array of photosensors being sensitive to a first color and thesecond linear array of photosensors being sensitive to a second colordifferent from the first color.